The human eye is much more sensitive to the yellow/green's in the specitrum than we are to the deep blues. The actinic bulbs have the majority of their light output at or near the fringe of the human eye's sensitivy. This is why the actinic bulbs look less intense as far as output as comapired the same wattage bulb that produces more yellow/green in its specitrum.

With that said adding another 250W bulb will make the tank look more white than it does now probably. This wont wash out the color of the corals though. Right now I suspect the are directly under the bulb is a very bright white light and the fringes of the tank are more blueish in tint from the actinics. The second bulb will make the tank more white overall as far as the light tint.

I have 250 watt mh. I originally had 10,000k and one burned out. I was going to replace with 14,000k for both. One of those burned out within a few days so I now have one of each. That being said, my point is that the 14,000k bulb has a much whiter/blue light and the 10,000k is almost yellow in comparison. Therefore, I am pretty sure that the kelvin rating also has something to do with the visual spectrum to the human eye.